Cylinder head milling

Been reading, reading. Anyway I came across some interesting articles on milling cylinder heads or the block deck surface and from what I've read that for every 0.010" removed that .0048" needs to be removed from the intake side of the cylinder head in order to maintain proper intake port alignment. The China wall was mentioned but I don't remember what was said. I don't know if any of this is fact, how much of this is true. One machine shop did say that if a total of 0.040" or more is milled that the intake side has to be done, again I'm not speaking from experience and I don't know this as fact.